1. Field of the Invention
The present invention relates to a dehumidifying device intended for connection to an expiratory line of a ventilator before an expiratory flow meter.
2. Description of the Prior Art
The task of a ventilator, or a respirator, is to supply a patient with a breathing gas or anesthetic gas. This may occur in intensive care, in surgery when anesthesia is administered to the patient, to support spontaneous breathing or to impose a respiratory rate on a patient incapable of spontaneous respiration. The flow of expiratory gas is measured in the ventilator's expiratory section as a part of the ventilator's control function.
Sometimes a bacteria filter is also connected to the expiratory section to prevent the patient's bacteria and virus from escaping into ambient air. Since gas from the patient is expired gas, its temperature is relatively high, about 30.degree. to 35.degree. C., and the gas is saturated with moisture, i.e. its relative humidity is 100%.
This high humidity can cause major problems due to water condensation in the flow meter and bacteria filter. Condensation in the bacteria filter makes it more difficult for the patient to breathe, since condensate increases resistance in the expiratory section. One prior art flow meter is described in the Operating Manual for the Servo Ventilator 900 C, Siemens-Elema AB, publication no. AG 0888 12, Aug. 1988, pp. 10:2-10:6. This flow meter contains a mesh through which expired air flows from the patient. If the relative humidity of the patient's expired air is too high when passing the mesh, condensation could form on the mesh, thereby disrupting the flow meter function sufficiently to produce an erroneous measurement of air flow and thereby affect the operation of the entire ventilator.
Another type of flow meter is described in Bulletin PP101, Gould Godart Fleish Flow Transducers, Gould Inc. This flow meter has a system of channels through which the gas runs. Determination of the pressure on both sides of the channel system provides a measure of the flow from the pressure gradient. At a high relative humidity, water could condense in the channel and obstruct the channel with an ensuing increase in the pressure gradient across the channel system, leading to an erroneous value for flow.
When such a fault occurs, the flow meter must be removed from the ventilator, cleaned, dried, reinstalled and calibrated. Since calibration is particularly important and cannot be performed with a patient connected to the ventilator, one flow meter cannot merely be replaced with another. The entire ventilator must be replaced. This naturally creates needless, irritating extra work for staff. Therefore, avoiding this type of error is particularly desirable.
A drop of approximately 20% in humidity (moisture content) is sufficient to guarantee flow meter operation. Although such a goal may appear simple to achieve, many different methods and designs have been tried over the years in attempts to solve the problem. One such design is described in the aforementioned Operating Manual for the Servo Ventilator 900C, page 1:3, and is based on heating the flow meter to about 60'C in order to reduce the relative humidity. As is well-known, the ability of air to hold water varies directly with its temperature.
Another solution to the problem is described in the brochure Star Exhalation Isolation System, Operating Instructions, Siemens version, Infrasonic Inc., form no. 9910053, Apr. 1988. A device is connected to the ventilator's expiratory inlet to heat expired air passing through the device; the device is also equipped with a water trap to collect condensed water.
Yet another solution employs a cooling device for condensing water vapor in expired gas before the gas is fed into the ventilator's flow meter. The device is described in the article "Expired Gas Cooling Device" by J. Attwood and L. Bartel, of the U.S. However, the different methods and designs have the disadvantage of either being inadequate in reducing relative humidity sufficiently or being bulky, clumsy units which have to be connected to the ventilator or which are dependent on a reliable power supply. This means that greater reserve power capacity or batteries would be needed in the event of a power failure or when a portable ventilator is used. Additional equipment also increase the staff's work load when connecting the patient, and equipment must be monitored the entire time the patient is connected.
The equipment's bulky design is also a problem, especially when patients have to be transported while connected to the ventilator.